i.e. make double call of func(func(f)) to return exactly the same as func(f).

This is correct to do as the first func call already returns a wrapper
that setups additional frame for defer. The second func call, if doing
the same, will wrap the thing just one more time and there will be two
frames for defer, but defer needs only one to work correctly.

So far we had no case when such double func calls would appear in
practice, because

    @func
    @func
    def f():
         ...

would immediately catch attention.

However in the next patch we will have this case to appear internally
when handling properties. So it is better to make sure beforehand no
waste of resources will happen.

/reviewed-by @levin.zimmermann
/reviewed-on nexedi/pygolang!31